Well. classically it isn't poorly defined

As that is the way we observe the vacuum normally.

Heh.

But yes JP, I agree. If we add all other effects we find, as the Casimir force, then it definitely is 'something' more than only 'empty'. That is me assuming that the Casimir force is a result of hidden 'energy', and not a result from matter of course. Maybe this can be considered a proof though?

Moving mirrors. It's also interesting to note that in 'General Relativity, as the modern theory of the gravitation, the gravitative effect of vacuum-energy results in the cosmological constant Λ' according to some sources. Although

"Although the existence of the vacuum-energy is proven, its energy-density is still unclear today. The value of the energy-density is regarded as the largest discrepancy in modern physics. As an average over several literature references of cosmology, the energy-density can be estimated at about (9.0±0.27)·10

^{-10}J/m

^{3}, whereas in Geometrodynamics a value of h·c·π/Lp

^{4}=3.32·10

^{+113}J/m3 is suspected.

However the latter value is calculated by an integration over all wavelengths of the quantum mechanical zero point oscillations within the vacuum (these are infinitesimally many), whereby divergence problems are suppressed simply by the means of cut-off radii. Several other approaches to suppress the divergence problems of these improper integrals (leading to the energy density) result in further other values for the energy-density of the vacuum, but they do not solve the problem of the ambiguity." From

Definite Proof for the Conversion of vacuum-energy into mechanical energy based on the Measurement of Machine Power.And no, the later I will not use as a proof, but it seems interesting still?